"20/20" continues, with the children who break away. Once again, David Muir. Reporter: After four months at home, paralyzed by fear, afraid other students at school are contaminated, Michelle tries to go back to school one class at a time. Science class is first. I felt like I need to, like, get back at the OCD and feel like I can do it. Reporter: She's crossed the first big hurdle. She's there in class. But right away, within minutes of the first period, we notice Michelle is already anxious about something, asking about the eighth grade prom and an overnight trip to Hershey park. But you're going to Hershey, right? Reporter: It's a trip that is still two months away. But she can't imagine an overnight trip away from home. She's just trying to get through a day at school, science, shop class and lunch in the chaotic cafeteria. It's a little weird because the kids are, like, you know, like, where have you been? Reporter: Even the principal seemed uncertain about just how to handle Michelle on her first day back. I put up my hand to kind of do a high five, and she didn't blink, and she high-fived me, and I knew we were on the right track. Reporter: Michelle has made it back to school. But next comes the real homework, the next battle in her fight against OCD. After school, she goes to Kohl's, the one department store she fears most. I'm, like, feeling anxious already just thinking about going. Reporter: Not because of the store itself, but because of the other students she has seen there. Am I gonna be able to pull into the parking lot? Yes, you can pull into the parking lot. Okay. Can you stick your hand out the window? What's happening? The germs are coming through. How does that feel? Icky. Reporter: Michelle knows her fear is irrational, but that is OCD. What's going through your head, Michelle? That I'm dirty. I'm gonna get out. No. I'm just gonna get out and stand right here. Reporter: Mom gets out of the car and then reaches down to touch the pavement. She then holds onto her daughter's hand to show her that nothing bad will happen. See, I touched the ground, you touched me. Reporter: And now, the hard part, actually walking toward the store, to the entrance that Michelle hasn't walked through in over four months. The store is contaminated from the kids at school. Reporter: What about the people out there who might say, you know, we're enabling these children and a little tough love would probably be better, just push them into the department store or -- Think about when you're afraid of anything. If someone tries to push you against your will harder than you feel ready to, your tendency is to dig your heels in, to say, you know, to just pull back. They want to get better. Do you want to walk back and forth? Just on that line. The idea is that once you face your fear and you face it over and over and over again over a period of time, you become less sensitive to it. You can't go and run and touch the door and run back? Okay. I won't push. You did good. How do you feel? I feel good. Reporter: A hug from her mother. A mini milestone before they head home for the day. While back in the therapist's office, Bridget is still trying to get home. For her, it's been six months living away from her family. And Bridget is about to face her next fear -- her mother now on the couch. Next comes her father. You're 15. He's missed half of that year. He's waiting. I'm scared. Reporter: Her father walks in. But they ask us that we not show his face because he works undercover in law enforcement. She cannot make eye contact with her own father. When you feel ready, I'd like you to try to unball yourself. Reporter: We sit in the room for 35 minutes. Bridget still trying to calm down. She still can't even look in the direction of her own father. And then, the small steps begin. Bridget musters the courage to look at her father's shoes. They are brown and black and have laces. Reporter: The doctor asks Bridget to look beyond the shoes directly at her father. A full hour has gone by. She never looks. Do you want to finish for the day? Yes. Do you want to finish for the day? Reporter: Her dad leaves the room. And we wondered, will she ever break through? Will she ever go home? Or will the OCD win? Dr. David Rosenberg is convinced Bridget and the other children like her will conquer it. His team at Wayne state and the dmc children's hospital of Michigan is on the front lines of this fight against child anxiety and OCD. In a groundbreaking study looking at the mris of children who have OCD, they can actually see it. So these are all mris of children with OCD? Exactly right. Reporter: They're convinced there's a genetic component here, and that much of this comes down to the amount of a certain chemical in the brain. In a child who has OCD, you see something in the brain. There's too little glutamate in certain areas of the brain, there's too much glutamate in other areas of the brain. Reporter: The doctor says glutamate controls the brain's light switch. And when the balance is off, the switch short-circuits. So the part of brain that gives most of us the all clear when we're worried never gives the child that all-clear. And instead of getting the signal that "Okay, I'm safe now," children with OCD get the signal that things are getting much more dangerous and unsafe. Reporter: Many of us check the doors when we leave the house, or the stove, sometimes two or three times. Eventually, though, our brain gives us the message it's okay to move on. Children with OCD and severe anxiety never get that message. Instead of getting the all clear, which is what the rest of us get? Exactly right. Reporter: And tonight, here's something else. The amount of the chemical and where they find it in the brain can tell them which kind of OCD your child might have. Findings he just revealed in new York City. So a child who washes their hands over and over again is not the same as a child who checks the locks. We see different things in the brain, and they respond differently to treatment. Reporter: He says it's a distinction his team was never able to make until now. So Michelle and Bridget are more contamination fears. Have a different brain scan than Rocco. So this is more Rocco? That's more likely to be Rocco. My name is Elizabeth, and I have obsessive compulsive disorder. Reporter: Remember Elizabeth? She was the teenager who once punched holes in her wall. She got help, exporapy and medicine. And since that home video, she became the national spokesperson for the obsessive compulsive foundation. How long did it take for you to get ready this morning? I'd say 30, 40 minutes. With a typical girl, how long it'd take a typical girl to get ready. Reporter: That's a huge feat. Yeah, I can remember taking five to six-hour showers. Reporter: And if she could do it, we wondered, what about Rocco? During our five year journey, we've checked in with him. One visit took us to New York's central park. He'd been on antidepressant medication and therapy, and both were working. On this morning, he did struggle to get out of the house, but he got here and on to that merry-go-round. 17-1-1. Reporter: 17-1-1. He boasts of his baseball team's record, but his parents know the biggest victory was just getting him here after all those mornings they couldn't get him out the front door. Even for Rocco, thinking back is painful. By putting a name to it, it helped Rocco understand, "Hey, I have OCD. Okay, and this is how I'm gonna

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